Magnetic switching

Methods of exerting magnetic forces to collect and manipulate magnetic particles disposed in a portion of subsurface vasculature using a wearable device are provided. The wearable device is configured to change the exerted magnetic force over time. For example, the exerted magnetic force could be sufficient to collect the magnetic particles during a first period of time and low enough to release the magnetic particles during a second period of time. The exerted magnetic force could be changed over time to vary some effect on the magnetic particles, for example to control a rate of release of collected magnetic particles. In some embodiments, the magnetic particles are configured to bind to an analyte of interest. The collection and manipulation of the magnetic particles can enable detection of one or more properties of the analyte, modification of the analyte, and/or extraction of the analyte bound to the magnetic particles.